Surface impurity-enriched diamond and method of making

ABSTRACT

An element-doped diamond crystal is disclosed herein. The crystal includes at least one dopant element which has a greater concentration toward or near an outermost surface of the crystal than in the center of the crystal. The concentration of the dopant element is at a local minimum at least about 5 micrometers below the surface. The concentration-profile of the dopant element for these diamond crystals causes an expansion of the diamond lattice, thereby generating tangential compressive stresses at the surface of the diamond crystal. These stresses beneficially increase the compressive fracture strength of the diamond.

[0001] This patent application is a continuation-in-part of applicationSer. No. 08/748,246, filed on Nov. 12, 1996.

TECHNICAL FIELD

[0002] This invention is related to a diamond composition and article,grown at high temperature and pressure with improved compressivefracture strength (CFS). More particularly, this invention relates tothe inhomogeneous incorporation of selected elements toward and/or nearthe outer surface of the diamond crystal to increase its fracturestrength and performance. The invention also relates to methods formaking diamond crystals which are surface-enriched with elements thatincrease their compressive fracture strength.

BACKGROUND OF THE INVENTION

[0003] Diamond crystals are typically used for the production of diamondtools, such as grinding wheels, dressing or truing tools for grindingwheels, and saw blades.

[0004] To function effectively in the above-mentioned applications,diamond crystals having the highest strength, including compressivefracture strength, are desirable. The compressive fracture strength is akey specification for diamond crystals, and is often correlated with thediamond's performance in such applications. Crystal fracture strength,as measured by Roll Crusher, correlates with performance of diamond inthese applications, such as stone cutting.

[0005] The compressive fracture strength may be measured bycompressively fracturing a population of diamond grains both before andafter treatment. The Roll Crusher method utilizes an apparatus having apair of hard counter-rotating rollers adapted with a means to measurethe compressive force applied by the rollers to grains passing betweenthe rollers at the moment of grain fracture. The motion of one of therollers is measured by a suitable transducer, such as a linear voltagedifferential transformer, to generate an electric signal proportional involtage to the deflection of the roller, and hence, proportional to thecompressive force on the diamond grain.

[0006] Diamond is a brittle solid and fails by fracture. Its elasticconstant (Young's modulus of diamond) is high: 1.143×10¹² pascals. Theductile-brittle transition temperature for diamond is about 1150° C. Theabove applications use diamond in the brittle fracture region extendingfrom room temperature to about 1000 ° C.

[0007] Impurity atoms dissolved in the diamond crystal, such as boron,nitrogen, and hydrogen, can generate stresses because of the largelattice dilations that they cause. For example, dissolved nitrogen,boron, and hydrogen atoms expand the diamond lattice around them by 40%,33.7%, and 31%, respectively. If the distribution of dissolved nitrogen,boron, or hydrogen is uniform in a diamond crystal, the lattice justexpands uniformly, and no long-range stresses develop. However, if thedistribution of nitrogen, boron, or hydrogen is not uniform,inhomogeneous strains will occur in the diamond. These uneven strainswill generate large long-range stresses.

[0008] Presently, the impurity concentration in synthetic diamondcrystals decreases with increasing radius within the diamond crystal.There are several reason for this. For instance, in the High PressureHigh Temperature (HPHT) process used to synthesize diamond crystals, theconcentration of nitrogen in the melt decreases with time because thegrowing diamond takes up nitrogen from the melt. (“Melt” refers to themolten, metallic catalyst/solvent through which carbon is transportedfrom the graphite feedstock to the growing diamond crystals at highpressure and high temperature.). Another reason is that the growth rateof the diamond crystal is decreasing with increasing radius, and theimpurity incorporation decreases with decreasing growth rate. Further,the impurity concentration in the diamond crystal may depend on thegrowth sector. By “growth sector” is meant the crystallographicdirection in which growth took place (or is taking place), and theregion(s) in the crystal in which growth occurred in the same direction.

[0009] This decreasing impurity concentration with increasing radius(i.e., negative concentration gradient) causes tangential tensilestresses on the surface of the diamond. Since diamond is a brittlesolid, its compressive fracture strength is reduced by these tangentialtensile stresses.

[0010] As technology advances, the next generation of diamond willrequire higher strength. Thus, there is a need for a diamond crystalwith increased compressive fracture strength. There is also a need formethods to manufacture these diamond crystals.

SUMMARY OF THE INVENTION

[0011] In accordance with these needs, this invention comprises anelement-doped diamond crystal, comprising at least one dopant elementwhich has a greater concentration toward or near an outermost surface ofthe crystal than in the center of the crystal, wherein the concentrationof the dopant element is at a local minimum at least about 5 micrometersbelow the surface. Examples of the dopant elements are boron, nitrogen,hydrogen, lithium, nickel, cobalt, sodium, potassium, aluminum,phosphorous, oxygen, and any mixture thereof.

[0012] Usually, the dopant-element concentration within an outermostsection of about 3-50 micrometers of the diamond crystal is in an amountof about 40 to about 10,000 parts per million. Moreover, in someembodiments, the concentration of the dopant element is at a localmaximum at a distance less than about 5 micrometers from the surface ofthe crystal.

[0013] The concentration-profile of the dopant element for diamondcrystals of this invention causes an expansion of the diamond lattice,thereby generating tangential compressive stresses at the surface of thediamond crystal. The generation of tangential compressive stressesincreases the compressive fracture strength of the diamond, as comparedto a diamond crystal in which the diamond lattice is not substantiallyexpanded. The increase in compressive fracture strength is preferably atleast about 2%, and more preferably, at least about 4%. The increase incompressive fracture strength is especially useful for synthetic diamondapplications and articles of manufacture, such as diamond grit, which isused in cutting or grinding tools.

[0014] Methods for making diamond crystals having a relatively highcompressive fracture strength, along with the impurityconcentration-profile described herein, are also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a plot of the growth rate versus the diamond crystalradius for the prior art.

[0016]FIG. 2 also shows the prior art, depicting a plot of nitrogenimpurity level in parts per million (ppm), versus the diamond crystalradius.

[0017]FIG. 3 also represents the prior art, depicting the plastic yieldstress (solid line) of diamond, versus temperature. The diamond-graphiteequilibrium line (dashed line) is also shown.

[0018]FIG. 4 also represents the prior art, depicting the radial,tangential, and shear stresses in a diamond with a negative, linearradial gradient of impurities.

[0019]FIG. 5 demonstrates this invention, depicting the radial,tangential and shear stresses in a diamond with a positive, linearradial gradient of impurities.

[0020]FIG. 6 also demonstrates this invention, depicting the radial,tangential and shear stresses in a diamond with a thin, impurity-richshell contiguous to the outer surface of the diamond.

[0021]FIG. 7 is a photomicrograph of a (111 ) crystal facet on a diamondcrystal of this invention, having a thin, impurity-rich shell on thesurface of the diamond.

[0022]FIG. 8 is a photomicrograph of a (100) crystal facet on a diamondcrystal of this invention, having a thin, impurity-rich shell on thesurface of the diamond.

[0023]FIG. 9 is a photomicrograph of a cross-section of the coatingthickness of the diamond of this invention.

DETAILED DESCRIPTION OF THE INVENTION

[0024] The present invention is based in part on the discovery that apositive radial gradient (i.e., increasing concentration with increasingradius) of an impurity element within a diamond lattice producestangential compressive stresses at the diamond surface. These stressesstrengthen the diamond in the same manner as tempered glass. (This typeof spatial distribution of impurities causes a greater expansion of thediamond lattice near the surface of the crystal, than near the center ofthe crystal, which in turn produces the tangential compressive stressesat the crystal surface).

[0025] Examples of the impurity or doping element include boron,nitrogen, hydrogen, lithium, nickel, cobalt, sodium, potassium,aluminum, phosphorous, and oxygen. Also, interstitials and vacancies inthe crystal may be considered impurities, including carboninterstitials. Boron, hydrogen, and nitrogen readily incorporate intothe diamond during growth. In this invention, nitrogen can have a localconcentration of about 0-1000 parts per million (ppm); boron can have alocal concentration of about 0-10,000 ppm; and hydrogen can have a localconcentration of about 0-1000 ppm. Moreover, there can be a mixture ofboron, nitrogen and hydrogen with a concentration of about 0-10,000parts per million.

[0026] Many of the diamond crystals of the present invention would notbe considered semiconductors, e.g., p-type semiconductors. For example,when the dopant element is boron, aluminum, or a mixture thereof, theconcentration of nitrogen which is dissolved in diamond crystals of thisinvention may exceed the total concentration of the dopantelements—often by at least about 5 ppm. In such an instance, the diamondcrystal generally cannot function as a semiconductor. As anotherexample, when the dopant element is nitrogen, hydrogen, nickel, cobalt,oxygen, or a mixture thereof, the diamond crystal also cannot generallyfunction as a semiconductor.

[0027] Natural isotopic abundance diamond is comprised of two carbonisotopes, namely 98.9% ¹²C carbon and 1.1% ¹³C carbon. More than 1.1% ofcarbon isotope ¹³C can be considered an impurity in natural isotopicabundance diamond. The ¹³C isotope causes a contraction of the diamondlattice. Therefore, a negative concentration gradient of ¹³C isotope isproduced in a diamond crystal, and compressive tangential stresses willbe developed at the surface of the diamond, causing the diamond to bestrengthened.

[0028] The diamond crystal utilized in the invention is a syntheticdiamond usually made by a High Pressure high Temperature process whereinpressures greater than about 45 kilobar are combined with temperaturesexceeding about 1200° C. in the carbon P-T region where diamond is thethermodynamically preferred phase. A variety of diamond crystals can beproduced. The chemical vapor deposition (CVD) method of making diamondscan also be used in this invention.

[0029] It is also contemplated that the process of the present inventionis suitable for use with a variety of starting diamond crystals (alsoreferred to as “grains). The commercially available diamond abrasivegrains are available at various grades of strength and toughness.Variations in growth rate during manufacture is one factor that may beutilized to control the properties of the diamond abrasive grain. Aperfect diamond crystal is likely to have more regular cleavage andwear. Crystal faults, twinning, and the like may occur and lead tovariations in the diamond crystalline properties.

[0030] The starting diamond crystals preferably have uncoated surfaces.It is intended that the process of the present invention be utilized toincrease the compressive fracture strength of the diamond prior tosubsequent attachment to a tool. By way of example, a diamond crystal ofthis invention is at least nominally a single crystal, but also may haveone or more twin planes present. It is a three dimensional (3-D) faceteddiamond crystal. The diameter of the diamond crystal is up to andincluding about 2 centimeters (2 cm). The diamond crystal has animpurity concentration (i.e., the concentration of the dopant element)that usually increases with increasing radius, leading to a compressivetangential stress at the surface of the crystal. In one embodiment ofthis invention, the impurities are at a local maximum at or near thesurface of the diamond crystal, decreasing with smaller radii to a localminimum at least about 5 micrometers below the surface of the crystal.(In other words, the local maximum and local minimum could be very closetogether). The impurity concentration within the outermost 3-50micrometers is about 10 to about 10,000 parts per million. (Takenliterally, the “outermost” surface region of the crystal would bemeasured from the actual crystal surface in a radial direction into thecrystal. However, for the present disclosure, the region up to about 3micrometers into the crystal is not taken into account for the purposeof measuring dopant-element concentration, since that region is affectedby the phenomenon of “catalyst imprints”, as described below).

[0031] Also, this invention includes a 3-D diamond crystal in which onecomponent (i.e., the tangential component) of the stress state of someof the facet surfaces of the crystal is compressive in the range ofabout 10 to about 5000 megapascals (MPa). The tangential compressivesurface stresses are up to and including about 5000 MPa. The radialsurface stress is always 0 MPa. Also, the compressive tangentialstresses on the diamond crystal of this invention can be superimposed onexisting tensile tangential stresses on the crystal. The range ofsuperimposed compressive tangential stresses on preexisting tensiletangential stresses in the diamond crystal is about 10 to about 5000MPa.

[0032] Herein, the invention will be demonstrated by using boron,nitrogen and hydrogen. Because of the extremely high binding energy andatomic density of diamond, only boron, nitrogen and hydrogen candissolve to an appreciable extent in diamond. Even in the cases ofboron, nitrogen and hydrogen, the solubility is limited, and amounts toless than a few hundred parts per million for nitrogen and hydrogen, andto less than 1% for boron. Despite their limited solubilities indiamond, dissolved nitrogen, boron or hydrogen can generate stressesbecause of the large lattice dilations that they cause. If thedistribution of nitrogen, boron or hydrogen is not uniform,inhomogeneous strains will occur in the diamond. These uneven strainswill generate large long-range stresses. An approximate estimate of themagnitude of these stresses can be found by multiplying the Young'smodulus of diamond (10¹² Pascals) by the strain generated by animpurity. For nitrogen in diamond, the maximum strain occurs at themaximum solubility of nitrogen in diamond, and is of the order of about4.5×10⁻⁴. This amount of strain will result in a stress of the magnitudeof about 5×10⁸ Pascals. This compares with experimental diamond crushingstrengths of about 0.2×10¹⁰ to about 2×10¹⁰ Pascals. Thus, stressesgenerated by non-uniform distributions of nitrogen in diamond can be ashigh as about 2.5% to about 25% of the crushing strength of diamond.

[0033] Uneven distribution of nitrogen or boron in diamonds can arisefrom two different sources. The first source of an inhomogeneousdistribution of boron or nitrogen is the variation of the distributioncoefficient (the ratio between the solubility of an impurity in theliquid and the solid phase) of boron and nitrogen with the diamondcrystal growth facet. For example, nitrogen incorporation typically isgreater in the (111) growth sector, slightly less in the (100) growthsector, and still less in the (113) and (110) sectors. The nitrogenconcentration is commonly as high as several hundred parts per million.If a section is made through the center of a typical synthetic diamondcrystal, the cross-section will typically exhibit both (111 ) and (100)growth sectors. The greater lattice expansion caused by nitrogen in the(111) growth sectors is incompatible with the lower lattice expansion inthe low nitrogen (100) sector. These incompatible strains generatetensile stresses in the (100) growth sector, and compressive stresses inthe (111 ) growth sector. The tensile stresses in the (100) sector mayweaken the diamond crystal in mechanical grinding and sawingapplications.

[0034] Boron also has a distribution coefficient that is dependent onthe crystal growth facet of diamond. For the higher concentrations ofboron that are of interest, boron is preferentially incorporated in the(111) sector in both gem diamonds and CVD diamond. Boron causes thediamond lattice to expand by 33.7%, and its solubility in diamond can beas much as 0.9%. Thus, the incorporation of boron will also lead tostresses in a diamond crystal in a manner similar to that of nitrogen.For boron in diamond, the maximum strain occurs at the maximumsolubility of boron in diamond, and is of the order of 3×10⁻³. Thisamount of strain can result in a stress of the magnitude of 3.4×10⁹Pascals, which is significant in comparison to the experimental diamondcrushing strengths of about 0.2×10¹⁰ Pascals to about 2×10¹⁰ Pascals.Thus, stresses generated by non-uniform distributions of boron indiamond can be as high as about 17% to 170% of the crushing strength ofdiamond.

[0035] A second source of an inhomogeneous distribution of boron ornitrogen in a diamond crystal arises from changes in the growth-cellconditions occurring during crystal growth. Temporal changes in thepressure, the temperature, the concentration of impurity in the meltand/or the crystal growth rate can change the amount of boron ornitrogen incorporated in the crystal as a function of time.Inhomogeneities arising from such causes will lead to radial impurityconcentration gradients in the crystal. The presence of stressesresulting from such radial concentration gradients suggests possibleways to toughen a diamond crystal by suitable changes in the growth-cellconditions during crystal growth.

[0036] To achieve an analytic solution, approximate the facetedsoccer-ball shape of an equiaxed diamond crystal by a diamond sphere.This is actually a reasonably good approximation to the highest qualitydiamond grit whose shape approaches that of a sphere. In addition,consider in detail only the case where the impurity concentrationgradients are solely a function of the radial distance R from the centerof the diamond crystal.

[0037] The mechanical equilibrium of any small spherical element demandsa balance between the differential radial forces on the opposing facesof the element. If Φ is the angle subtended in both tangentialdirections by the spherical element; σ_(r) is the radial stress, σ_(t)is the tangential stress, and R is the radius of the spherical element,then a balance of radial force requires:

[(R+dR)dΦ] ²[σ_(r)+(dσr/dR)dR]−(RdΦ)² σr=2[RdΦdR ]σ _(t) dΦ  (1)

[0038] which reduces to equation 2 as dR and dΦ approach zero.

dσ _(r) /dR+2(σ_(r)−σ_(t))/R=0  (2)

[0039] Strains in the diamond are caused by stresses, and by thepresence of impurities that cause expansion or contraction of thediamond lattice. The strain ε_(i) caused by an impurity is given by:

ε_(i) =αC,  (3)

[0040] where α is a constant dependent on the impurity, and C is theatomic concentration of the impurity in the lattice. For nitrogen, boronand hydrogen in diamond, α=0.4, 0.337 and 0.31, respectively.

[0041] The stresses developed in the diamond are related to the strainsby Hooke's Law:

σ_(r)−2νσ_(t) =E(ε_(r)−ε_(i))  (4a)

σ_(t)−ν(σ_(t)+σ_(r))=E(ε_(t)−ε_(i))  (4b)

[0042] where ν is Poisson's ratio, E is Young's modulus for diamond,ε_(r) is the radial strain, and ε_(t) is the tangential strain. Theimpurity strain ε_(i) in equations 4a and 4b is subtracted from theelastic strains ε_(r) and ε_(t), respectively, because the impuritystrain is not supported by elastic stresses, but instead is generated bychanges in the lattice size caused by an over- or under-sized impurityatom. The commonly-accepted value of Poisson's ratio for the diamond is0.07. The orientation-averaged Young's modulus for diamond is 1.143×10¹²N/m².

[0043] Equations 4a and 4b can be solved for the radial stress σ_(r) andtangential stress σ_(t):

σ_(r) =E/[(1+ν)][(1−ν)ε_(r)+2νε_(t)−(1+ν)αC]  (5a)

σ_(t) =E/[(1+ν)(1−2ν)][νε_(r)+ε_(t)−(1+ν)αC]  (5b)

[0044] The radial ε_(r) and tangential strains ε_(t) can be expressed interms of the radial displacement u.

ε_(r) =du/dR  (6a)

ε_(t) =u/R  (6b)

[0045] Equations 6a and 6b are substituted into Equations 5a and 5b, andthen into Equation 2, to give:

d ² u/dR ²+2/R du/dR−2u/R ²=(1+ν)/(1−ν)αdC/dR  (7a)

[0046] or

d/dR[1/R ² d/dR(R ² u)]=(1+ν)/(1−ν)αdC/dR  (7b)

[0047] which is Laplace's Equation in spherical coordinates. Integrationof Equation 7b gives the displacement u(R):

u(R)=(1+v)/(1−v)α1/R ² ∫C(r)r ² dr+A R+B/R ²,   (8)

[0048] where A and B are constants of integration, and C(r) is theconcentration of nitrogen or boron in the diamond, as a function of theradius. For a solid sphere of diamond by symmetry, the displacement u→0,as R→0, so that B=0. Equation 8 can be substituted back into Equations6a, 6b, 5a and 5b to find the radial and tangential stresses in thediamond:

σ_(r)(R)=−(2αE)/(1−v)1/R ³ ∫C(r)r ² dr+EA/(1−2v)   (9a)

σ_(t)(R)=\F(αE, (1−v))\F(1,R ³)∫C(r)r ² dr+EA/(1−2v)−αE C(R)/(1−v)  (9b)

[0049] The radial stress α_(r) at the outer free surface (α_(r)=0 atR=S) of the diamond must be zero (α_(r)=0 at R=S), so from Equation 9a,the constant A is found to be:

A=[2α(1−2v)/(1−v)](1/S ³)∫C(r)r ² dr,   (10)

[0050] where S is the radius of the diamond. Introduction of theconstant A in equations 9a and 9b yields:

σ_(r)(R)=(2αE)/(1−v)[1/S ³ ∫C(r)r ² dr−1/R ³ ∫C(r)r ² dr]  (10a)

[0051] Equation 10a has an interesting physical interpretation. Theradial stress is proportional to the difference between the averageimpurity concentration in the entire diamond (the first integral ofEquation 10a) and the average impurity concentration in the diamondwithin the radius R of interest (the second integral of Equation 10a).If the average concentration within a radius of R is greater than theaverage impurity concentration for the entire diamond, the radial stresswill be compressive for an impurity that expands the lattice.

[0052] The three cases demonstrated are: a radially increasingconcentration of impurity; a radially decreasing concentration ofimpurity; and a thin impurity-rich layer or “shell”. These differentimpurity distributions will generate different stress states in thediamond crystal, and may strengthen the crystal.

[0053] The following discussion is related to the stresses that developin a diamond crystal with particular radial distributions of impurities.

[0054] The first case of interest is where the concentration of impurityincreases linearly with the radius, i.e., C(r)=C_(o)r/S from r=0 to r=S.Substitution into Equations 10a and 10b yields:

α_(r)(R)=αEC _(o)/2(1−ν)[1−R/s]  (11a)

α_(t)(R)=αEC _(o)/4(1−ν)[1−3R/S]  (11b)

[0055] The following is the case where a is positive, that is, theimpurity causes an expansion of the diamond lattice. The radial stressis tensile, i.e., positive, and a maximum at the center of the diamond,and decreases linearly to zero at the surface of the diamond. Thetangential stresses are also a maximum at the center of the diamond, andare also tensile there. However, at a radius equal to ⅔ the radius ofthe diamond, the tangential stresses pass through zero and thereafterbecome compressive stresses, i.e., negative, and reach a maximumcompression state at the surface of the diamond. These tangentialcompressive stresses at the surface of the diamond will make the diamondless likely to fail in tension and fracture, and therefore toughen thediamond crystal.

[0056] The case where the impurity causes a contraction of the diamondlattice (α<0) just reverses the signs of the stresses, so that theradial stress is compressive, and is at an absolute maximum at thecenter, and decreases linearly to zero from the center to the surface ofthe diamond. The tangential stresses are also compressive and at anabsolute maximum at the center of the crystal. At a radius of ⅔ theradius of the diamond, the tangential stresses change from compressiveto tensile, and reach a maximum at the surface of the diamond. Thesetangential tensile stresses at the surface of the diamond may weaken thediamond by making it more susceptible to tensile failure and fracturefrom minor scratches.

[0057] Shear stresses can also cause a diamond to fail, particularly-athigher temperatures, where the shear yield strength rapidly decreaseswith increasing temperature. The maximum shear stress α_(s) in a diamondwith a radial-type of concentration gradient is given by one-half thedifference between the radial stress α_(r) and the tangential stressα_(t):

α_(s)(R)=1/2[α_(r)(R)−α_(t)(R)]=[αEC _(o)/8(1−ν)]R/S  (12)

[0058] It should be noted that the shear stress increases with theradius, and reaches a maximum at the surface of the diamond. Diamondwill fail when the shear stress exceeds the shear yield stress K ofdiamond, as shown in FIG. 3, which is dependent on the crystallographicdirection, crystallographic plane, and temperature.

[0059] The concentration of impurities and the radial, tensile and shearstresses are plotted as a function of the radius in FIG. 5, for the casewhere an impurity like boron or nitrogen expands the diamond lattice.

[0060] A second case of interest (demonstrating the prior art) is wherethe concentration of impurity decreases linearly with the radius, i.e.,C(r)=C_(o) (1=r/S) from r=0 to r=S. Substitution into Equations 10a and10b yields:

α_(r)(R)=−αEC ₀/2(1−ν)[1−R/S]  (13a)

α_(r)(R)=−αEC _(o)4(1−ν)[2−3R/S]  (13b)

[0061] The absolute value of the stresses remains unchanged, and onlytheir signs are changed from the previous case. Since the first term inthe concentration expression, C(r)=C_(o)(1−r/S), namely C_(o),represents a uniform concentration term, it does not cause any stresses.The expression —C_(p)(r/S) then remains, which is the same concentrationdistribution that was used in the previous case, except for its sign.

[0062] In the usual case for diamond, where α is positive, the impuritycauses an expansion of the diamond lattice. The radial stress iscompressive, i.e., negative, and a maximum at the center of the diamond,and decreases linearly to zero from the center of the diamond to thesurface of the diamond. The tangential stresses are also a maximum atthe center of the diamond, and are compressive there. However, at aradius equal to ⅔ the radius of the diamond, the tangential stressespass through zero and thereafter become tensile stresses, i.e.,positive, and reach an absolute maximum at the surface of the diamond.These tangential tensile stresses at the surface of the diamond willmake the diamond more likely to fail in tension and weaken the diamondcrystal by making it more susceptible to fracture.

[0063] The case where the impurity causes a contraction of the diamondlattice (α<0) just reverses the signs of the stresses so that the radialstress is tensile and at an absolute maximum at the center, anddecreases linearly to zero from the center to the surface of thediamond. The tangential stresses are also tensile and at an absolutemaximum at the center of the crystal. At a radius of ⅔ the radius of thediamond, the tangential stresses change from tensile to compressive, andreach a maximum at the surface of the diamond. These tangentialcompressive stresses at the surface of the diamond may strengthen thediamond by making it more resistant to tensile failure and fracturecaused by surface flaws or scratches.

[0064] Because only the sign of the radial and tangential stresses wasreversed by inverting the impurity expansion to impurity contraction,the shear stresses which are derived from the difference in the radialand tangential stresses remain the same as in the previous case. For theradially decreasing concentration gradient case, the concentration ofimpurities and the radial, tensile and shear stresses are plotted as afunction of the radius in FIG. 4 (prior art), for the case where animpurity like boron or nitrogen expands the diamond lattice.

[0065] A third case of interest is where the impurity resides in a thinshell of thickness “x” adjacent to the external surface of the diamond.Such a case could arise if the diamonds were exposed to boron, nitrogen,or hydrogen during the final stage of growth, so that the boron,nitrogen, or hydrogen becomes incorporated within a thin outer shell ofthe diamond crystal. Similarly, if the diamonds were exposed to a highenergy plasma containing hydrogen, nitrogen, or boron, these impuritiescould become implanted in a thin shell contiguous to the outer surfaceof the diamond.

[0066] This distribution of impurities would be described mathematicallyas follows: for 0<R<(S−x), C=0; for (S−x)<R<S, C=C_(o), where x is thethickness of the shell. Substitution into Equations 10a and 10b yieldsfor the condition of a thin shell, i.e., x<<S:

α_(r)(R)=2αEC _(o)/(1−ν)[x/S]:R<(S−x); x<<S  (14a)

α_(r)(S)=0:R=S;  (14b)

α_(t)(R)=2αEC _(o)/(1−ν)[x/S]:R<(S−x);x<<S  (14c)

α_(t)(S)=−αEC _(o)/(1−ν)[1−3x/S]:R=S;x<<S  (14d)

[0067] In the case where α is positive, that is, the impurity causes anexpansion of the diamond lattice, the radial stress is a small constanttensile stress in the zone inside the diamond which is free ofimpurities. The radial stress rapidly decreases as the outer shell ofimpurities is crossed, and falls to zero on the surface of the diamond,as expected. The tensile stresses are also small tensile stresses in theimpurity-free zone inside the diamond. However, the tangential stresseschange to large compressive stresses on the surface of the diamond. Thiscompressive stress state on the diamond surface will make the diamondmore resistant to tensile failure and fracture, and will thereby toughenand strengthen the diamond. FIG. 6 shows the impurity concentrationprofile and stress distribution for a diamond crystal with a thin shellof impurities at its outer surface.

[0068] In the limit of a thin coating (x<<S),

α_(t)(S)β−(αEC _(o))/(1−υ)  (14d')

[0069] It can easily be shown that this is a general result for diamondcrystals with a thin, impurity-rich shell, e.g., where the impurityconcentration decreases from C_(o) at the surface, linearly orexponentially with depth below the surface. The physical reason for thisis clear: the underlying crystal forces the thin shell to adapt the samelattice constant in the tangential direction, generating a compressivetangential stress, because the element-doped thin shell would otherwiseadopt a larger lattice constant.

[0070] The case where the impurity causes a contraction of the diamondlattice (α<0) just reverses the signs of the stresses. The case where adiamond containing a uniform level of impurity has a thin shell depletedof impurities contiguous to the external surface would have the sameabsolute stress values as Equations 14a-14d, but the signs of all thestresses would be reversed.

[0071] The shear stresses α_(s) generated by a thin shell of impuritiesare given by the difference between the radial stress and the tangentialstresses in the diamond:

α_(s)(R)=0:R<(S−x);x<<X  (15a)

α_(s)(S)=|α|EC/2(1−ν)[1−3x/S]:R=S;x<<S  (15b)

[0072] The shear stresses are zero inside the impurity-free region wherea state of pure hydrostatic tension exists. On passing through the thinspherical shell of impurities, the shear stresses rise to a maximum atthe surface of the diamond.

[0073] Equations 14 and 15 (i.e., the various equations under thosedesignations) demonstrate that the tangential stress at the surface ofthe crystal approaches a limiting value as the element-doped shellbecomes infinitesimally thin. However, there is a practical limit on thethickness of the shell, if it is to generate substantial stresses. Thefacets of diamond crystals grown at high pressure and high temperatureare not perfectly smooth. Instead, they are covered by shallow,vein-like structures, which are often referred to as “catalystimprints”. These structures are produced at the end of the growth cycleduring cooling, due to precipitation of carbon from the catalyst/solventas it freezes onto the surface of the diamond crystals. These vein-likefeatures are normally a few micrometers in height (usually less thanabout 3 micrometers). For a thin, impurity-containing shell to generatesubstantial tangential stresses at the surface of the crystal, it mustcontiguously cover the entire crystal, regardless of the presence of thecatalyst imprint, and therefore must be thicker than about 3micrometers.

[0074] The rate of linear crystal growth normally decreases during thecourse of growth, and impurities in the growth medium normally becomeincorporated into the diamond throughout the run. In one method of thisinvention, using the High Pressure High Temperature (HPHT) process tomake diamond crystals, a powdered boron source, such as boron carbide(B₄C) boron nitride (BN), or elemental boron, is encapsulated by carbonand added to the growth cell. Encapsulation of the boron source powderis achieved by CVD through methane pyrolysis; coating with colloidalgraphite and drying; sputtering; or the like. During the early stages ofdiamond growth at HPHT, no boron is available to growing diamondcrystals. As growth proceeds, the carbon coating dissolves into themetal solvent-catalyst, eventually exposing the boron source. At thispoint, boron begins dissolving in the solvent-catalyst, and becomesincorporated into the outer portion of the diamond crystals. The boronlevel and the thickness of the boron-doped layer can be controlled byvarying the concentration and size of the boron-containing particleswithin the growth medium, and the thickness of the carbon coat.

[0075] Another method is using a carbon-encapsulated nitrogen source,such as Fe_(x)N, to produce crystals with a positive nitrogen radialconcentration gradient. If desired, the nitrogen level in the centerportion of the crystals can be reduced by means of a getter in thegrowth medium, a low-porosity cell, or by excluding air from the growthcell immediately prior to growth.

[0076] Another method is to reduce the growth temperature for thecrystals by at least about 10° C. near the end of the HPHT cycle. Atleast about 5 micrometers of diamond is then grown at the reducedtemperature, followed by quenching. Impurity incorporation decreaseswith increasing growth temperature. Therefore, the reduced, final growthtemperature will generate a higher impurity concentration near thesurface of the crystal.

[0077] Another alternative method of the invention involves depositionof a boron or nitrogen-rich layer on lightly- or undoped diamondcrystals grown normally. This could be performed by using the undopedcrystals as seeds in a second HPHT run, where sufficient boron,nitrogen, or boron and nitrogen is present in the growth medium toproduce a doped, epitaxial layer over the crystal. This option requiresa second growth run. However, since only a thin shell (about 3 to about50 micrometers thick) is needed, the number of diamond crystals in thecell could be larger than normal.

[0078] Still another method of the invention involves diffusingimpurities into the diamond crystals by means of exposure to a plasmawhich contains hydrogen, nitrogen, or boron, or some mixture of any ofthose gasses. Plasma equipment is well-known in the art.

[0079] In an alternative method, the undoped or lightly-doped crystalscould be placed in a CVD (chemical vapor deposition) reactor, and adoped, epitaxial layer grown by adding a boron or nitrogen-containinggas to the normal reactant feedstock—typically hydrogen, methane andoxygen. Deposition at a substrate temperature less than or approximatelyequal to 740° C. would add additional intrinsic compressive stress tothe doped diamond layer, and could perform the purpose of thisinvention, even in the absence of doping. The film of doped diamond isabout 3 to about 50 micrometers thick, and the concentration of thedoped impurity in the outer layer is greater by about 40 to about 10,000parts per million than the concentration of the impurities in theoutermost portion of the underlying diamond crystal.

[0080] One way that crystal growth conditions can generate stresses in adiamond crystal is the case where the pressure and temperature in thecell vary with time. If controlled in a suitable manner, such varyingconditions can produce positive radial impurity concentration gradientsin the diamond crystal. For impurities such as boron, nitrogen orhydrogen that expand the diamond lattice, positive radial gradients willproduce a compressive stress state at the surface of the diamond thatwill toughen the diamond and make it more resistant to tensile fracture.Also, by increasing the growth rate of the diamond crystal as a functionof the radius (time), there will be an increase in the incorporation ofthe concentration of impurities in the diamond. Furthermore, byincreasing the concentration of impurities in the melt with time, therewill be an increasing impurity concentration-versus-radius.

[0081] Yet another method to achieve the diamond crystal with increasedCFS of this invention is to produce swelling near the outer surface ofthe diamond crystals grown by the High Pressure High Temperature process(HPHT). This can be done by radiation damage by means of ion or electronbombardment. The radiation damage causes swelling of the diamond latticebecause of interstitials and vacancies, where the interstitial atomscould be either carbon atoms or impurity atoms, or a combinationthereof. This thin shell of lattice-expanding impurities will generate acompressive stress state at the surface of the diamond crystal, and makeit more resistant to fracture. In contrast to the strengthening providedby radial gradients, the protection from this thin compression shelldisappears once the thin shell is penetrated by abrasion of the crystalduring use. A 3-D diamond crystal having a surface which has beenion-implanted or subjected to radiation damage may have an impurityatom, interstitial, and vacancy concentration in the range of about1-10,000 parts per million.

[0082] Other methods for doping impurities in the outermost surface ofthe diamond crystal include impurities deposited in a layer byhydrothermal growth, electrochemical growth, liquid phase solid sourcegrowth, or molten salt growth.

[0083] Table 1 demonstrates the invention and the impact of impurityconcentrations of nitrogen, boron and hydrogen on the compressivefracture strength (CFS) for a diamond crystal at its outer surface.TABLE 1 Strength Increase for Diamond Crystal With InhomogeneousConcentration of Impurities (Dopant Elements) Near the Surface MaximumMaximum Tangential Tangential Maximum Stress Stress Increase Conc.(Graded) (Thin Layer) in Strength Impurity (ppm) (MPa) (MPa) (Graded)(Thin) Nitrogen 1,000 123 492 7.7% 30.8% Boron 10,000  1040 4150 65.0% 259% Hydrogen 1,000 95 381 5.9% 23.8%

EXAMPLES

[0084] The following examples further serve to illustrate the invention.

Example 1

[0085] The starting material was a sample of high-grade diamond crystalsobtained from General Electric Company (GE Superabrasives). The crystalshad a 40/45 mesh size, and were grown according to the HPHT process,which is known to those skilled in the art. The nitrogen distribution inthe untreated crystals showed a tensile tangential stress at the surfaceof approximately 50 MPa, weakening the crystals. The compressivefracture strength (CFS) of the untreated crystal was measured byroll-crusher apparatus as 63.5 lbs.

[0086] The nitrogen (N) concentration was measured as 4.2 ppm on the(100) crystal facet by Secondary Ion Mass Spectrometry (SIMS) (not anabsolute value). The nitrogen (N) concentration was measured as 29.5 ppmon the (111) crystal facet by SIMS (also not an absolute value). TheSIMS signals were converted to concentrations by using ion-implanteddiamond standards. However, ion implantation causes lattice damage, andlikely affects the sensitivity. Comparison of the SIMS data withnitrogen concentrations determined by combustion analysis (LECO) ofwhole crystals indicates that the SIMS-determined concentrations are toolow by a factor of 2-4.

[0087] The diamond crystals were then placed on a substrate inside amicrowave plasma chemical vapor deposition (CVD) reactor. The flow ratewas 1000 sccm hydrogen (H₂), 5 sccm methane, and 10 sccm nitrogen (N₂).The pressure was about 70 torr; the substrate temperature was betweenabout 700-750° C., and the microwave power was about 3 Kilowatts. Thediamond crystals were coated with nitrogen-doped CVD diamond for 3hours. The coating growth was interrupted to roll the crystals around,and then the coating deposition was repeated for another 3 hours toachieve a uniform coating.

[0088] The thickness of the nitrogen-doped CVD diamond shell was about 5micrometers. The compressive fracture strength (CFS) of the coatedcrystals was measured by a roll-crusher apparatus at 65.9 lbs—anincrease of 3.8%. The nitrogen concentration in the diamond layer on the(100) face of the coated crystals was measured by SIMS as 62.5 ppm (notan absolute value). The nitrogen (N) concentration on the (111) face ofthe coated crystals was measured by SIMS as 79.5 ppm.

[0089] Using the average surface concentrations measured by SIMS, thetheoretical model of the present invention predicts that the coatinggenerated a compressive tangential stress of 26.6 MPa, resulting in anincrease in fracture strength of 1.7%. However, scaling theSIMS-determined surface nitrogen concentrations by a factor of 2-4, asmentioned above, would predict a strength increase of 3.4-6.8%, inexcellent agreement with the measured value of 3.8%.

[0090] Pock marks, surface roughness (see FIGS. 7 and 8), unevenness ofthe coated layer (also see FIG. 9) gives somewhat lower strengtheningthan calculated.

Example 2

[0091] The starting material was another sample of high-grade diamondcrystals obtained from General Electric Company. The crystals had a40/45 mesh size, and were grown according to the HPHT process, as inExample 1. The nitrogen distribution in the untreated crystals showed atensile tangential stress at the surface of approximately 50 MPa,weakening the crystals. The compressive fracture strength (CFS) of theuntreated crystals was measured by roll-crusher apparatus at 70.6 lbs.

[0092] The diamond crystals were then placed on a substrate inside amicrowave plasma chemical vapor deposition (CVD) reactor. The flow ratewas 900 sccm hydrogen (H₂). The pressure was about 100 torr; thesubstrate temperature was between about 700-900° C., and the microwavepower was about 3 Kilowatts. The diamond crystals were exposed to thehydrogen plasma for 2 hours. The hydrogen plasma process was interruptedto roll the crystals around. The hydrogen plasma process was thenrepeated two times for 2 additional hours each time, to achieve uniformexposure of the crystals to the hydrogen plasma.

[0093] Compressive fracture strength (CFS) of coated crystals wasmeasured by roll-crusher apparatus as 75.7 lbs—an increase of 7.2%.

[0094] Based on literature reports of hydrogen incorporation intodiamond by means of hydrogen plasma treatments (M. I. Landstrass et al.,Appl. Phys. Lett. 55, 975 (1989); T. Maki et al., Jpn. J. Appl. Phys.31, L1446 (1992); G. Popovici et al., J. Appl. Phys. 77, 5103 (1995)),the surface hydrogen concentration is estimated to be as high as 1000ppm. The theoretical model of the present invention predicts thesurprising result that the indiffused hydrogen atoms will generate acompressive tangential stress as high as 380 MPa, resulting in anincrease in fracture strength of 24%. The observed strength-increase iswell within this range.

Example 3

[0095] A comparison was made between the teachings of U.S. Pat. No.3,268,457 (Giardini et al) and the teachings of the present invention.“Giardini” discloses a method of creating electrically semiconductingdiamonds. The reference was quite dated, and did not provide adescription of some of the process conditions which would be helpful incomparing its teachings with those of the present disclosure. However,process conditions (e.g., pressing time) for Giardini which would appearto be most favorable for producing the diamond crystals of the presentinvention were selected.

[0096] Eight samples were used, each containing diamond crystals. SampleA served as a control, and contained only diamond crystals, which werenot treated in any way. (The diamond crystals themselves weresubstantially identical to the diamond crystals used for treatment inExample 1). Sample B served as a second control. It contained a mixtureof 0.1 g of diamond crystals and 2.3 g of graphite powder, but did notcontain any dopant-impurity. (The graphite serves as a type of pressuremedium, which was apparently its function in the Giardini patent).Sample C contained diamond crystals and graphite powder in the sameratio as sample B, but further contained 5% by weight ammonium chloride,based on the weight of diamond and graphite. This sample was designed tofollow Example 6 of Giardini, i.e., the use of ammonium chloride as animpurity. Sample D contained diamond crystals and graphite powder in thesame ratio as sample B, but also contained 5% by weight boron carbide,based on the weight of diamond and graphite. This sample was designed tofollow Example 9 of Giardini, i.e., the use of boron carbide as animpurity. All of the diamond crystals used for samples A-D had a 40/45mesh particle size. (Particle size was not a significant issue for theseexperiments. The diamond crystals for the second set of samples (E-H)simply originated from a different batch.).

[0097] Samples E, F, G and H were analogous to samples A, B, C and D(respectively), except that these diamond crystals had a 45/50 meshparticle size. Thus, samples E and F were controls. Sample G was treatedwith ammonium chloride, while sample H was treated with boron carbide.

[0098] Using a punch-and-die apparatus, each blend (samples B, C, D, F,G, and H) was pressed (10,000 lb. force) into a pill having a diameterof 0.44 inch, and a height of 0.5 inch. The two pills having the samegeneral composition (i.e., samples B and F; C and G; D and H) werestacked together in a cell formed by two facing cups made of magnesiumoxide. The cells were in turn placed inside sleeves fabricated fromsodium chloride. The cells were pressed for 20 minutes at a pressure ofabout 50 killobar, and a temperature of about 1500° C. (based onstandard calibration methods). Power to the pressure device was turnedoff. The samples were then cooled, and the pressure was released. Thepills were removed from the cell, and the diamond was separated from thegraphite by boiling the material in a mixture of concentrated nitricacid and sulfuric acid.

[0099] The compressive fracture strength (CFS) of each sample (includingsamples A and E) was immediately measured by a roll-crusher apparatus,as described in the examples for the present invention. TABLE 2 SampleCompressive Fracture Strength A*; E* (Untreated diamond 66.2 lbs 59.7lbs. crystals) B**; F** (Diamond crystals 70.5 lbs. 59.9 lbs. andgraphite) C; G (Diamond crystals 65.0 lbs. 57.6 lbs. and NH₄Cl) D; H(Diamond crystals 62.7 lbs. 60.4 lbs. and B₄C)

[0100] The results in Table 2 show that diamond crystals treatedaccording to the teachings of Giardini showed little, if any, increasein compressive fracture strength. While sample H appeared to show a verysmall increase (within experimental error), treatment-of the diamondcrystals with two notable impurity sources (samples C and D: ammoniumchloride and boron carbide, respectively) resulted in decreases in CFSvalues: decreases of 1.9% and 5.3%, respectively, as compared to theuntreated diamond crystals of sample A. This is in marked contrast todiamond crystals treated according to the present invention, asdescribed in Examples 1 and 2. In those examples, the CFS valuesincreased considerably when the diamond crystals were treated accordingto this invention. In the case of doping with nitrogen (Example 1), theincrease in CFS was 3.8%. In the case of doping with hydrogen (Example2), the increase in CFS was 7.2%. A considerable increase in CFS forboron would also be expected, when such a dopant is used to treatdiamonds according to the process of the present invention. Moreover, anincrease in CFS would also be expected when the dopant was lithium,nickel, cobalt, sodium, potassium, aluminum, phosphorous, oxygen, ormixtures thereof, using this process.

Example 4

[0101] The following experiment is also relevant to a comparison of thepresent invention with the teachings of the Giardini patent discussed inExample 3. In an attempt to diffuse hydrogen into diamond crystals, 0.20g of high-grade, 40/45 mesh diamond crystals were placed in a boronnitride (BN) cup. 0.14 g of molten paraffin wax was added to the cup andallowed to cool. A BN lid was placed on top of the cup, and the cup wasplaced in a high-pressure cell and treated at a pressure ofapproximately 55 kbar and a temperature of approximately 1500° C., for60 minutes. After cooling and depressurization, the crystals wereremoved from the cell and isolated by heating in a mixture ofconcentrated nitric acid and sulfuric acid.

[0102] Visually, the crystals appeared the same as before the treatment.The compressive fracture strength (CFS) of the untreated andparaffin-annealed crystals was measured, using the roll-crusherapparatus mentioned above. The annealing treatment reduced the CFS from54.5 lbs to 45.3 lbs, i.e., a decrease of 17%. While experimentalconditions were not equivalent to those described in Example 3 (e.g.,graphite was not used here, and the pressure levels were slightlyhigher), the conditions were sufficiently close to the teachings of theGiardini patent to permit certain observations. First, standardtreatments based on prior art knowledge about doping diamond crystalsare not generally capable of strengthening the diamonds in the mannertaught by the present invention. Second, the lack of strengthening insuch crystals is a strong indication that the crystals lack one or moreof the required features of diamond crystals of the present invention,e.g., the impurity profile mentioned above and described fully herein.

[0103] Moreover, the technology described in the Giardini patent is notsufficient to control the diffusion of impurities into diamond crystalsto any desired value. While the Giardini technology may be able tocontrol the diffusion of carriers to some extent, it would not generallybe capable of providing the impurity profile claimed in the presentinvention. Moreover, the Giardini crystals would not be expected toexhibit the tangential compressive stress values which appear tocharacterize the diamond crystals of the present invention.

Example 5

[0104] Some of the diamond crystal samples prepared in Example 3 (i.e.,containing ammonium chloride as a source for nitrogen impurities, orcontaining boron carbide as a source of boron impurities) were subjectedto a sputter depth-profile analysis. The analysis was carried out withthe SIMS technique, using a Cameca 3F Ion Microscope. (The samplescorrespond to the sample numbers for Example 3.) In the sputteringtechnique, cesium was used as the primary ion beam, at a beam energy of14.5 keV. The raster size was about 100 micrometers, and the detectedarea was approximately 30 micrometers.

[0105] The dimensions in the table refer to depth or distance into thediamond crystal, from the surface. The results are separated fornitrogen content and boron content. TABLE 3 Mean, 0-2 μm Depth of LocalMean, 3-5 μm Sample* (ppm) Minimum (μm) (ppm) Nitrogen A-(100) 18.5 20.7 B-(100) 12.7 — 11.9 C-(100) 55.1 2 2.1 D-(100) 111.2 2.6 2.5 A-(111)4.2 — 4.8 B-(111) 13.2 1.3 2.9 C-(111) 51.2 1.0 41.2 D-(111) 15.1 0.511.6 Boron A-(100) 0.03 — 0.03 B-(100) 0.04 0.4 ** C-(100) 0.07 0.9 **D-(100) 0.03 — ** A-(111) ** — ** B-(111) 0.14 0.5 ** C-(111) 0.07 0.5** D-(111) 0.42 0.6 **

[0106] The data in Table 2 and Table 3 demonstrate clearly that diamondcrystals treated according to the Giardini process fail to exhibit thedopant/impurity profile of diamond crystals of the present invention.While some of the Giardini-like samples do display a local minimum forthe particular impurity concentration, none of the samples had a localminimum which occurred at a depth of 3 microns or greater.

[0107] The Giardini-like crystals also fail to exhibit the increase instrength (e.g., CFS) shown with diamond crystals of the presentinvention. In the case of doping with nitrogen, the Giardini crystalsdid not appear to have a maximum impurity concentration at the surfaceof the crystal, with decreasing concentration toward the center of thecrystal. In the case of doping with boron, the Giardini crystals mayhave exhibited a maximum impurity concentration at the crystal surface,but the concentration of dopant in that region was too small toappreciably strengthen the crystal.

[0108] Having described preferred embodiments of the present invention,alternative embodiments may become apparent to those skilled in the artwithout departing from the spirit of this invention. Accordingly, it isunderstood that the scope of this invention is to be limited only by theappended claims.

[0109] All of the patents, articles, and texts mentioned above areincorporated herein by reference.

What is claimed:
 1. A three-dimensional faceted diamond crystal,comprising at least one dopant element which has a greater concentrationtoward or near an outermost surface of the crystal than in the center ofthe crystal, wherein the concentration of the dopant element is at alocal minimum at least about 5 micrometers below the surface.
 2. Thediamond crystal of claim 1 , wherein the dopant element is selected fromthe group consisting of boron, nitrogen, hydrogen, lithium, nickel,cobalt, sodium, potassium, aluminum, phosphorous, oxygen, and mixturesof any of the foregoing.
 3. The diamond crystal of claim 2 , wherein thedopant element is selected from the group consisting of boron, nitrogen,hydrogen, and mixtures of any of the foregoing.
 4. The diamond crystalof claim 1 , wherein the dopant element is boron, aluminum, or mixturesof any of the foregoing.
 5. The diamond crystal of claim 4 , whereinnitrogen is dissolved in the crystal, and wherein the concentration ofnitrogen exceeds the total concentration of dopant elements.
 6. Thediamond crystal of claim 5 , wherein the concentration of nitrogenexceeds the total concentration of dopant elements by at least about 5parts per million.
 7. A diamond crystal according to claim 1 , whereinthe dopant element is selected from the group consisting of nitrogen,hydrogen, nickel, cobalt, oxygen, and mixtures of any of the foregoing.8. A diamond crystal according to claim 1 , wherein the dopant-elementconcentration within an outermost section of about 3 to about 50micrometers of the crystal is in an amount of about 40 to about 10,000parts per million.
 9. A diamond crystal according to claim 1 , whereinthe concentration of the dopant element is at a local maximum at adistance less than about 5 micrometers from the surface of the crystal.10. A diamond crystal according to claim 1 , wherein the concentrationof the dopant element causes an expansion of the diamond lattice towardor near the outermost surface of the crystal, thereby generatingtangential compressive stresses at the surface of the diamond crystal.11. The diamond crystal of claim 10 , wherein the tangential compressivestresses are in the range of about 10 to about 5000 megapascals (MPa).12. The diamond crystal of claim 10 , wherein the generation oftangential compressive stresses increases the compressive fracturestrength of the diamond, as compared to a diamond crystal in which thediamond lattice is not substantially expanded.
 13. The diamond crystalof claim 12 , wherein the increase in compressive fracture strength isat least about 2%.
 14. An article of manufacture which comprises thediamond crystal of claim 1 .
 15. A three-dimensional faceted diamondcrystal containing at least one dopant element, wherein theconcentration of the dopant element is greater toward or near theoutermost surface of the crystal than in the center of the crystal, saidcrystal being further characterized by a tangential compressive stressat a surface of said crystal of up to about 5000 megapascals.
 16. Thediamond crystal of claim 15 , having a diameter up to about 2centimeters.
 17. The diamond crystal of claim 15 , wherein said crystalis a single crystal.
 18. The diamond crystal of claim 15 , having one ormore twin planes.
 19. The diamond crystal of claim 15 , wherein thedopant element is selected from the group consisting of boron, nitrogen,hydrogen, lithium, nickel, cobalt, sodium, potassium, aluminum,phosphorous, oxygen, and mixtures thereof.
 20. The diamond crystal ofclaim 15 , comprising a coated film of doped diamond about 3 to about 50micrometers thick on an outer surface of the crystal, wherein theconcentration of the dopant element in the coated film is about 40 toabout 10,000 parts per million greater than the concentration of thedopant element in the outer surface of said underlying diamond crystal.21. The diamond crystal of claim 20 , wherein the dopant element isdiffused into the diamond crystal, and the concentration of the dopantelement is about 40 to about 10,000 parts per million at a depth ofabout 3 micrometers to about 50 micrometers within said diamond crystal.22. The diamond crystal of claim 15 , wherein a range of tangentialcompressive stresses of about 10 to about 5000 megapascals issuperimposed on preexisting tangential tensile stresses in said diamondcrystal.
 23. A method of making a diamond crystal having a tangentialcompressive stress on a surface up to about 5000 megapascals, comprisingthe step of: growing a three-dimensional diamond crystal by a HighTemperature High Pressure process, wherein said crystal includesimpurities that enrich an outer surface of the crystal in aconcentration that is about 40 parts per million to about 10,000 partsper million, at a depth from the outer surface of about 3 micrometers toabout 50 micrometers.
 24. The method of claim 23 , wherein the growthtemperature for the diamond crystals is reduced by at least about 10° C.after about 80% of the normal growth cycle is completed.